1. Field of the Invention
This invention relates to radiation curable coating compositions containing ethylenically unsaturated polyurethanes and substrates coated therewith.
2. Description of the Prior Art
In the past UV coatings were touted as being 100% solids. Unfortunately, in order to achieve the desired performance properties medium-high molecular weight acrylic functional oligomers with high viscosity were developed. These materials were too viscous to be used alone and were formulated with reactive thinners, which are low molecular weight esters of acrylic or methacrylic acid. These reactive thinners have an elevated risk of toxicity and are not desired.
Recently, in order to reduce the reactive thinner content, solvent has been used to reduce the viscosity of the formulation, thus reducing the need for substantial amounts of reactive thinner. In many cases, some reactive thinner is still used in order to not exceed VOC limits and to moderate the properties of the film.
The market has repeated asked for lower viscosity oligomers in order to reduce the amount of reactive thinner and solvent. As an example, U.S. Pat. No. 5,739,251 discloses one-component coating compositions containing ethylenically unsaturated polyisocyanates which contain allophanate groups and have i) an NCO content of 5 to 25% by weight, ii) a content of β,γ-ethylenically unsaturated ether groups incorporated through allophanate groups of 0.5 to 15% by weight and iii) an allophanate group content of 1 to 30%.
As another example, U.S. Pat. No. 5,777,024 discloses an allophanate-modified urethane resin formed by reacting an allophanate-modified isocyanurate with one or more hydroxy-functional olefinic compounds, which is used in coating compositions curable by high-energy irradiation, including UV/visible light or electron beam irradiation.
DE 2 914 982 discloses hardenable compositions that include urethanes that are liquid at room temperature or are solid at room temperature and can be melted at below 60° C., which are prepared by reaching esters or polyesters containing hydroxy terminal groups with difunctional and/or polyfunctional, and optionally subsequently with monofunctional, isocyanatoalkyl and/or isocyanatoaryl compounds.
However, the above-described radiation curable coating compositions typically require high amounts of reactive thinner for application and often provide films with insufficient toughness and/or poor flexibility. Thus, there is a need in the art for radiation curable coating compositions that require lower amounts of reactive thinner for application and yield tough, flexible films.